1. Field of the Invention
The present invention relates to an antistatic composite coating film applied on an easily electrostatically chargeable substance whereby to harden the surface of said substance so as to make it more difficult to injure the substance and also to impart an antistatic property to the substance.
2. Description of the Prior Art
Plastics are used widely because of their easy moldability, high impact resistance and light weight. However, plastics have the disadvantages that the surface thereof is easily spoiled or damaged and is easily electrostatically chargeable. The appearance thereof is deteriorated seriously by scratches on the surface thereof and dust adhered thereto and, in the case of transparent plastics, the transparency is reduced disadvantageously. In the cases of plastics to be used as window glass or optical lenses, they must have a hard surface which is difficult to damage, difficult to electrostatically charge by friction and difficult to soil, and the antistatic property should not be lost by washing with water and the transparency should be maintained. There has been no plastic material which satisfies all of those requirements.
For hardening the surface of a plastic to protect it from injury, there has been proposed a process wherein a film of a hydrolyzed condensate of an alkyltrialkoxysilane is formed on the surface of a plastic (for example, Official Gazette of Japanese Patent Laid-open No. 143822/1975). A sufficient antistatic property cannot be obtained by this process, although the resistance to marring is improved. It has also been known that a hydrolyzed condensate of silicon tetrachloride acts as an antistatic agent [Kobunshi (High Molecules), 10, 371 (1961)]. However, a film of a hydrolyzed condensate of silicon tetrachloride is hard and brittle so that a film thereof of appreciable thickness is sometimes cracked when an external stress is applied thereto. If a hydrolyzate of silicon tetrachloride is applied on a film of a sufficiently hardened hydrolyzed condensate of said alkyltrialkoxysilane and then it is hardened, the adhesion between the two films is weak and the antistatic property is deteriorated by ultrasonic washing with water. If a surfactant-antistatic agent is applied onto a hardened coating of hydrolyzed condensate of an alkyltrialkoxysilane, the antistatic agent is washed away by water whereby the antistatic property is lost. If a mixture of the hydrolyzate of alkyltrialkoxysilane, the silicon tetrachloride hydrolyzate and an antistatic agent such as a surfactant-antistatic agent, used in an amount effective for imparting antistatic property thereto, is applied to the surface of a plastic molded article and then is dried and thereby hardened, the resulting coating film is opaque, and the adhesion between the coating film and the substrate, the hardness of the surface and the mar resistance are reduced.
Thus, the antistatic treatments by the prior processes cause some defects and there has not been a completely satisfactory process. However, according to the process of the present invention, an antistatic composite film is formed which film has a high surface hardness and an excellent antistatic property which is not deteriorated by washing with water.